Devices of that kind for coating a metal strip are basically known in the prior art thus, for example, from DE 10 2009 051 932 A1. In concrete terms, this specification discloses a coating container filled with a liquid coating material. For coating, the metal strip is conducted through the container with the coating material. After leaving the coating container the metal strip runs through a blowing device, which is arranged above the coating container, for blowing excess parts of the still liquid coating material off the surface of the metal strip. An electromagnetic stabilising device for stabilising the metal strip after leaving the coating container and the blowing device is arranged above the blowing device and is supported by the blowing device. The electromagnetic stabilising device has the effect, in particular, that the strip is held centrally in a centre plane of the overall device and that oscillations of the metal strip during transit through the coating container and the blowing device are prevented or at least reduced.
Each of the blowing device and the electromagnetic stabilising device has a respective slot through which the metal strip is guided. In order to achieve a uniform thickness or thickness distribution of the coating material on the upper side and lower side of the metal strip it is essential for the metal strip to run in a predetermined target centre position through the slot of the blowing device. Only then is it guaranteed that the action of the blowing nozzles of the blowing device on the upper side and lower side of the metal strip is the same and a desired uniform thickness distribution of the coating material on the metal strip results.
The target centre position is defined by, in particular, a preferably uniform spacing of the wide sides and the narrow sides of the metal strip from the oppositely disposed nozzles of the blowing device and, in particular, by the fact that the metal strip is not inclined, twisted or too-strongly curved relative to the longitudinal orientation of the slot or the nozzles.
However, in practice it can happen that the metal strip after leaving the coating container is strongly curved. Such a curvature is, as stated, undesirable particularly for passage through the blowing device. Traditionally, the curvature is therefore counteracted by a correction roller being adjusted against the metal strip before the metal strip enters the blowing device. However, this has the disadvantage that as a consequence the actual position by which the metal strip runs through the slot of the blowing device can depart from the target centre position, which can lead to the above-described problem of a non-uniform thickness distribution of the coating.
German published specification DE 10 2007 042 897 A1 discloses a method with a device for coating a metal strip with a coating material, for example zinc. For this purpose, the metal strip is conducted through a coating container filled with the liquid coating material, in which case the coating material adheres to the surface of the metal strip. After leaving the coating container the metal strip runs through a slot of a blowing device which serves the purpose of blowing excess parts of the still liquid coating material off the surface of the metal strip. In addition, a curvature sensor is provided for detecting the actual curvature of the metal strip after leaving the coating container. Insofar as the detected actual curvature exceeds a predetermined permissible curvature threshold value, a correction roller is adjusted against the metal strip in order to smooth this. Control of the blowing device is also disclosed.
Specification DE 43 00 868 C1 teaches positioning of the blowing device with the help of adjusting drives so that the spacing between nozzle gap and strip surface remains constant. The corresponding control or regulation of the position of the blowing device is carried out in dependence on the actual position of the metal strip, which is continuously detected by a separate measuring device.
WO 94/02658 A1 teaches provision at least implicitly of a curvature sensor for detecting the curvature of the metal strip above the blowing device. If excessive curvature is ascertained so that there is a possible risk of contact between strip and blowing device the blowing device is moved transversely to the plane of the metal strip until a minimum spacing between metal strip and blowing device is reinstated over the entire width of the metal strip.
JP 2003 113460 A provides a so-called displacement sensor (numerical value), which is arranged in the electromagnetic stabilising device and which is formed to detect the state of curvature or the amount of eccentricity of the metal strip in the slot of the electromagnetic stabilising device. Depending on the amount of eccentricity of the metal strip current is supplied to the electromagnets of the electromagnetic stabilising device so as to generate magnetic forces on the metal strip of such a kind that the curvature and the path position of the metal strip are corrected. A stabilising roller and the correction roller are controlled or positioned in the metal bath with the help of a process control device in dependence on the output values. In addition, stripper nozzles are controlled and positioned in correspondence with a mathematically determined eccentricity index and, in particular, so that the index or eccentricity of the metal strip lies within or below a predetermined threshold value. The stripper nozzles and the electromagnetic units or the electromagnetic stabilising device are respectively moved in parallel by the same amount.
The invention has the object of developing a known method and a known device, of the kind stated in the introduction, for coating a metal strip in such a way that non-uniform thickness distribution of the coating on the metal strip is prevented by way of adjustment of the correction roller.